1. Field of the Invention
The present invention relates to a resin composition to be used as a material for a core portion of an optical waveguide, an optical waveguide produced by using the resin composition, and a production method of the optical waveguide.
2. Description of the Related Art
Optical waveguides are typically incorporated in optical devices such as optical waveguide devices, optical integrated circuits and optical wiring boards to be widely used for optical communications, optical information processing and other general optics. Such an optical waveguide typically includes core portions of a predetermined pattern provided as light paths, and an under-cladding layer and an over-cladding layer covering the core portions.
A roll-to-roll process, for example, is conceivably employed for production of the optical waveguide. Where an epoxy-based optical waveguide material (varnish) is used as a core material in the roll-to-roll process, for example, the varnish is applied onto a base film and then dried, whereby a double-layer film including an uncured core formation film layer is formed. When the double-layer film is used, the uncured core formation film layer obtained by the application and the drying of the varnish should satisfy the following property requirements for adaptation to the roll-to-roll process: (1) the uncured film layer is free from tackiness (or is tack-free) so as to be prevented from being transferred onto a roll of the double-layer film and from suffering from surface roughness; and (2) the film layer has flexibility so as to be prevented from being cracked due to a stress caused by warpage thereof during transportation of the roll of the double-layer film. Therefore, the material for the core formation film layer is required to contain no liquid component, but is required to contain a solid resin as a polymer component having a relatively high molecular weight to satisfy the property requirement (1).
In view of this, a solid epoxy resin, for example, is used as the solid resin. In order to cure the solid epoxy resin with excellent patternability through photo-cationic polymerization, a multifunctional epoxy resin having an epoxy group is used for improvement of curing sensitivity. However, the mobility of reactive species generated from a photoacid generator (PAG) blended with the solid epoxy resin is absolutely reduced as compared with a case in which a liquid epoxy resin is used. Therefore, the amount of the photoacid generator (PAG) required to be used for formation of a fine pattern of core portions arranged at a smaller pitch is inevitably increased as compared with the case in which the liquid epoxy resin is used. It is known that the photoacid generator (PAG) typically causes coloration (yellowing) of a product obtained by curing the resin. As a result, an optical waveguide produced by using a photosensitive resin composition adapted for the roll-to-roll process suffers from a higher optical waveguide loss than an optical waveguide produced by using a photosensitive resin composition not adapted for the roll-to-roll process. Thus, there is a limitation in reducing the optical waveguide loss, because the improvement of the patternability based on the amount of the photoacid generator (PAG) is contradictory to the reduction of the optical waveguide loss when the formulation of the core formation material is designed so as to be adapted for the roll-to-roll process.
On the other hand, it is known that a photosensitive resin composition containing a (meth)acrylate resin is typically cured with the use of a photoradical polymerization initiator, and an optical waveguide produced by using the photosensitive resin composition containing the (meth)acrylate resin generally has a lower optical waveguide loss than the optical waveguide produced by using the photosensitive resin composition containing the epoxy resin. To reinforce the thermal and mechanical fragility of the optical waveguide, a mixed material system containing both the (meth)acrylate resin and the epoxy resin is generally used (see, for example, JP-A-2009-175456 and JP-A-2009-175457).
However, the mixed material system containing both the (meth)acrylate resin and the epoxy resin as disclosed in JP-A-2009-175456 and JP-A-2009-175457 is not preferred because phase separation occurs between the (meth)acrylate resin and the epoxy resin to cause internal variations in haze and refractive index.
Further, the (meth)acrylate resin material experiences greater volumetric contraction than the epoxy resin material during a curing reaction. For various types of optical waveguides including a cladding layer required to be formed at higher dimensional accuracy, it is preferred, in consideration of various process steps, to form an over-cladding layer and an under-cladding layer from the epoxy resin material. If the core portions are formed from the (meth)acrylate resin material on the under-cladding layer formed from the epoxy resin material, however, the core pattern is liable to be separated or lost during a developing step in the formation of the core pattern. This is because adhesion strength between the under-cladding layer and the core portions is lower in the absence of chemical bonds between the under-cladding layer and the core portions. Therefore, the formation of the (meth)acrylate resin core portions on the epoxy resin under-cladding layer disadvantageously reduces the productivity.
In current technology for the production of the optical waveguide, there is an eager demand for an optical waveguide core material which is excellent in transparency as well as adhesion to the epoxy resin under-cladding layer, patternability and tack-free property for the adaptation to the roll-to-roll process, and reduces the optical waveguide loss.